This invention relates to an organic electroluminescent (EL) device and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
Active research works have been made on organic EL devices for use as displays because EL devices can be formed on a large area of glass. In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of tin-doped indium oxide (ITO) etc., a hole transporting layer of an organic amine compound, a light emitting layer of an organic fluorescent material exhibiting electronic conductivity and intense light emission such as an aluminum quinolinol complex (Alq3), and an electrode of a metal having a low work function such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. Structures having two or three organic compound layers are typical.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 4 and 5 illustrate typical configurations of organic EL devices.
In FIG. 4, a hole transporting layer 14 and a light emitting layer 15, both of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 5, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16, all of organic compounds, are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Reliability is a common problem to be solved for these organic EL devices. More particularly, organic EL devices in principle have a hole injecting electrode and an electron injecting electrode and need organic layers for effectively injecting and transporting holes and electrons from the electrodes, respectively. However, the organic materials of which the organic layers are formed are vulnerable during manufacture and have less affinity to the electrodes. Another problem is raised by the significantly accelerated degradation of organic thin films as compared with light emitting diodes (LED) and laser diodes (LD).
The EL devices emit light under the influence of an electric field. The function of a semiconductor layer constituting such an EL device is based on the radiative recombination of electron-hole pairs injected from a pair of electrodes into the semiconductor. Exemplary devices are light emitting diodes (LED) based on GaP and similar Group III-Group V semiconductors. Although these devices are effectively and widely utilized, their size is so small that it is not only difficult, but also uneconomical to apply the LEDs to large area displays. Several types of materials are known as the substitutes applicable to large area displays. Of these inorganic semiconductors, ZnS is most useful. The ZnS system, however, suffers from serious practical drawbacks including the lack of reliability. One exemplary mechanism associated with ZnS is deemed to be that carriers of one type are accelerated through the semiconductor under a strong electric field, to induce the local excitation of the semiconductor which is relaxed by radiative light emission.
It is known that of organic materials, simple aromatic molecules such as anthracene, perylene, and coronene are electroluminescent.
On practical use, these organic materials have the problems that they lack reliability like ZnS, and joining layers of these materials to current-injecting electrode layers is difficult.
The technique of depositing organic materials through sublimation leaves the problem that the resulting layers are soft and likely to recrystallize.
The technique of building up properly modified aromatic compounds by Langmuir-Blodgett method invites deterioration of film quality, dilution of active material, and an increase of manufacturing cost.
An EL device using anthracene is disclosed in U.S. Pat. No. 3,621,321. This device has the inconveniences of increased power consumption and low luminescence.
Another attempt to provide an improved device is U.S. Pat. No. 4,672,265 which discloses an EL device comprising a luminescent layer of a double layer structure.
However, the materials used in the double layer structure are organic materials having the above-mentioned inconveniences.
JP-A 10-92576 discloses an EL device comprising a semiconductor layer in the form of a thin dense polymer film composed of at least one conjugated polymer, a first contact layer adjoining a first surface of the semiconductor layer, and a second contact layer adjoining a second surface of the semiconductor layer. The polymer film of which the semiconductor layer is formed has a sufficiently low concentration of external charge carriers so that, when an electric field is applied between the first and second contact layers across the semiconductor layer with the second contact layer made positive relative to the first contact layer, charge carriers are injected into the semiconductor layer whereby the semiconductor layer emits light.
Conjugated polymers themselves are known, and their application to optical modulators, for example, is disclosed in European Patent Application No. 0294061. Polyacetylene is used as an active layer in a modulating structure between first and second electrodes. An insulating layer must be disposed between one electrode and the active layer so as to form a space charge region in the active layer providing an optical modulation effect. Nevertheless, the presence of the space charge region disables formation of electron-hole pairs that emit light through their decay. Therefore, such a structure fails to exert electroluminescence. The development of electroluminescence is utterly undesirable in European Patent Application No. 0294061 because the optical modulation effect is destroyed thereby.
To solve these problems, it is contemplated to utilize the advantages of both an organic material and an inorganic semiconductor material. Specifically, an organic/inorganic semiconductor junction is given by substituting an organic p-type semiconductor for the organic hole transporting layer. Such studies are disclosed in Japanese Patent No. 2636341, JP-A 2-139893, 2-207488, and 6-119973. However, it is difficult to design devices which surpass conventional organic EL devices with respect to luminescent performance and device reliability.
An object of the present invention is to provide an organic EL device capable of utilizing the advantages of both organic and inorganic materials and having a high efficiency, long lifetime, and low cost.
The present invention provides an organic electroluminescent device comprising a hole injecting electrode, an electron injecting electrode, and an organic layer participating in a light emitting function disposed between the electrodes, said organic layer including a light emitting layer comprising a conjugated polymer. In one embodiment, the device further includes an inorganic insulative hole injecting layer disposed between the light emitting layer and the hole injecting electrode. In another embodiment, the device further includes an inorganic insulative electron injecting layer disposed between the light emitting layer and the electron injecting electrode. The device may include both the inorganic insulative hole injecting layer and the inorganic insulative electron injecting layer.
Preferably, the inorganic insulative hole injecting layer contains as a main component an oxide represented by the formula:
(SixGel1xe2x88x92x)Oy
wherein x is from 0 to 1 and y is from 0.2 to 1.99. The inorganic insulative hole injecting layer may contain an element having a work function of 4.5 to 5.5 eV in addition to the main component. It may further contain up to 30 at % based on the main component of carbon or nitrogen or a mixture of carbon and nitrogen.
Preferably, the inorganic insulative electron injecting layer contains a metal oxide having an electron affinity of up to 3.5 eV as a main component and at least one of oxides, nitrides and carbides of silicon and germanium.
In the organic EL device of the invention, the conjugated polymer used in the light emitting layer is preferably poly(p-phenylene vinylene), abbreviated as PPV. The polymer film preferably has a generally uniform thickness in the range of 10 nm to 5 xcexcm. The conjugated polymer preferably has a semiconductor band gap in the range of 1 eV to 3.5 eV. Also preferably, the proportion of the conjugated polymer in an electroluminescent zone in the polymer film is sufficient to achieve the percolation threshold to ensure charge transfer within the conjugated polymer in the film.
The conjugated polymer designates a polymer having a nonlocalized xcfx80 electron system along its main skeleton. The nonlocalized xcfx80 electron system endows the polymer with semiconductive properties as well as an ability to carry positive and negative charge carriers having a high mobility along the polymer skeleton.
Such polymers are discussed by R. H. Friend in Journal of Molecular Electronics, 4 (1988), January-March, No. 1, pp. 37-46, for example.
In the organic EL device, the hole injecting electrode and hole injecting layer serve to inject positive charge carriers into the polymer film whereas the electron injecting electrode and electron injecting layer serve to inject negative charge carriers into the polymer film. These charge carriers are combined to form charge pairs susceptible to radiative decay. For this reason, it is preferred that the hole and electron injecting electrodes be selected so as to have high and low work functions, respectively.
To acquire the desired electroluminescence, the polymer film is preferably substantially free of defects which act as the center of non-luminescent recombination. Such defects obstruct the electroluminescence.
In addition to the charge injecting function, the inorganic insulative hole injecting layer or electron injecting layer, at least one of which is formed, serves to control the ratio of electrons to holes injected into the electroluminescent layer and to ensure that radiative decay occurs apart from the inorganic insulative hole injecting layer or electron injecting layer.
The film of conjugated polymer is preferably composed of a single conjugated polymer or a single copolymer comprising segments of a conjugated polymer. Alternatively, the film of conjugated polymer may be composed of a mixture of a conjugated polymer or copolymer and another suitable polymer.
Further preferred characteristics of the polymer film are given below.
(i) The polymer is stable upon exposure to oxygen, humidity and high temperature.
(ii) The polymer film has good adhesion to the underlying layer, an ability to inhibit the occurrence of cracks caused by temperature rise and pressure bias, and resistance to shrinkage, expansion, recrystallization or other morphological changes.
(iii) Owing to high crystallinity and a high melting point, for example, the polymer film is recoverable with respect to an ion/atom transfer step.